Plasticizers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, toys, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticizers include polyvinyl butyral, acrylic polymers, poly(vinylidene chloride), nylon, polyolefins, polyurethanes, and certain fluoroplastics. Plasticizers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers). A listing of the major plasticizers and their compatibilities with different polymer systems is provided in “Plasticizers,” A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp. 157-175.
Plasticizers can be characterized on the basis of their chemical structure. The most important chemical class of plasticizers is phthalic acid esters, which accounted for about 85% worldwide of PVC plasticizer usage in 2002. However, in the recent past there as been an effort to decrease the use of phthalate esters as plasticizers in PVC, particularly in end uses where the product contacts food, such as bottle cap liners and sealants, medical and food films, or for medical examination gloves, blood bags, and IV delivery systems, flexible tubing, or for toys, and the like. For these and most other uses of plasticized polymer systems, however, a successful substitute for phthalate esters has heretofore not materialized.
One such suggested substitute for phthalates are esters based on cyclohexanoic acid. In the late 1990's and early 2000's, various compositions based on cyclohexanoate, cyclohexanedioates, and cyclohexanepolyoate esters were said to be useful for a range of goods from semi-rigid to highly flexible materials. See, for instance, WO 99/32427, WO 2004/046078, WO 2003/029339, WO 2004/046078, U.S. Application No. 2006-0247461, and U.S. Pat. No. 7,297,738.
Other suggested substitutes include esters based on benzoic acid (see, for instance, U.S. Pat. No. 6,740,254, and also co-pending, commonly-assigned, U.S. Patent Application 61/040,480 and polyketones, such as described in U.S. Pat. No. 6,777,514; and also co-pending, commonly-assigned, U.S. application Ser. No. 12/058,397. Epoxidized soybean oil, which has much longer alkyl groups (C16 to C18) has been tried as a plasticizer, but is generally used as a PVC stabilizer. Stabilizers are used in much lower concentrations than plasticizers.
Typically, the best that has been achieved with substitution of the phthalate ester with an alternative material is a flexible PVC article having either reduced performance or poorer processability. Thus, heretofore efforts to make phthalate-free plasticizer systems for PVC have not proven to be entirely satisfactory, and this is still an area of intense research.
Plasticizers based on triglycerides have been tried in the past, but they have mostly been based on natural triglycerides from various vegetable oils. The alkyl groups on these natural triglycerides are linear, and can cause compatibility problems when the alkyl chain is too long.
“Structural Expressions of Long-Chain Esters on Their Plasticizing Behavior in Poly(vinyl Chloride”, H. K. Shobha and K. Kishore, Macromolecules 1992, 25, 6765-6769, reported the influence of branching and molecular weight in long-chain esters in PVC. Triglycerides (TGE's) having linear alkyl groups were studied.
“A Method for Determining Compatibility Parameters of Plasticizers for Use in PVC Through Use of Torsional Modulus”, G. R. Riser and W. E. Palm, Polymer Engineering and Science, April 1967, 110-114, also investigate the use of triglycerides and their plasticizing behavior with PVC, including tri-iso-valerin (3-methyl butanoate)triglyceride. It was reported that “these materials have volatilities that are much too high for good long-time permanence”.
Nagai et al. in U.S. Pat. No. 5,248,531, teaches the use of articles comprising vinyl chloride-type resins (among others) using triglyceride compounds as a hemolysis depressant, and also comprising 10 to 45 wt % of plasticizers selected from trialkyl trimellitates, di-normal alkyl phthalates, and tetraalkyl pyromellitates. The alkyl chains of the acid-derived moiety R1-R3 in the structure below, formula (I), are independently an aliphatic hydrocarbon group of one to 20 carbon atoms and in embodiments at least one of the alkyl chains is branched. One specific triglyceride disclosed is glyceryl tri-2-ethylhexanoate.

Zhou et al. discloses, in U.S. Pat. Nos. 6,652,774; 6,740,254; and 6,811,722; phthalate-free plasticizers comprising a mixture of different triesters of glycerin, preferably wherein the phthalate-free plasticizer is formed by a process of esterifying glycerin with a mixture comprising a mixture of alkyl acids and aryl acids. Zhou et al. also discloses that glyceryl tribenzoate and glyceryl tri(2-ethyl)hexanoate have not been used as primary plasticizers in vinyl polymers, such as PVC because they are known to be incompatible with such resins.
Nielsen et al., in U.S. Pat. No. 6,734,241, teach a composition comprising a thermoplastic polymer as in formula (I) above, wherein at least one of the R groups is an alkyl group having from one to five carbon atoms and at least one of the R groups is a saturated branched alkyl group having from 9 to 19 carbon atoms and a hydrophilic group.
Among the problems presented by the aforementioned triglycerides is they cannot be made conveniently and thus generally are quite expensive and/or are specialty chemicals not suitable as replacements for phthalates from an economic standpoint and/or are not as compatible with the range of polymer systems that phthalates are compatible with, and thus are not viable replacements for phthalates from a physical property standpoint.
For instance, some synthesis methods involve at least two separate steps, such as where the glycerol is first partially esterified with the C10 to C20 branched chain acyl group and then reacted with acetic acid or acetic anhydride.
Other syntheses involving mixed acid feeds will require addition of a hydrocarbon solvent for azeotropic distillation of the water to drive the esterification reaction to completion (as measured by the hydroxyl number of the ester, which is a measure of the amount of unreacted OH groups), due to the spread in boiling points between the mixed acids. In addition, the use of mixed acid feedstock such as cited in Zhou et al. and in Nielsen et al. can reduce the capability of recycling unreacted acids.
Triglycerides based on acids derived from natural products will be limited to naturally occurring linear alkyl groups with even carbon numbers, which offer very little flexibility in designing an appropriate plasticizer system for a given polymer system.
Thus what is needed is a method of making a general purpose non-phthalate plasticizer having high throughput and providing a plasticizer having a suitable melting or glass transition or pour point, increased compatibility, good performance and low temperature properties.
The present inventors have surprisingly discovered an improved process that yields triglycerides with a total carbon number of the triester groups between 20 and 25 (not including the three glycerol backbone carbons), produced by esterification of glycerol with a combination of acids derived from the hydroformylation and subsequent oxidation of C3 to C9 olefins, provide for triglycerides for excellent compatibility with a wide variety of resins and that can be made with a high throughput. Esterification of glycerol using a combination of acids eliminates many of the aforementioned problems, and enables high yield of the glycerol triesters to be obtained, having excellent compatibility with vinyl polymers.